Filtering method for water treatment

Abstract

The invention relates to a method for filtering water by means of a filter (10, 10'') comprising at least one granular medium forming a layer of granular media (14), which is submerged and placed in a tank (18), the filtering method comprising: - at least two steps of cleaning (2) the filter, the filtering method being characterised in that it comprises at least one step of unclogging (4) the granular media, comprising the following steps: - stopping the filtering of the water; - at least once, placing the grains forming the granular media in suspension by means of at least one injection of at least one unclogging fluid into the layer of granular media (14); - returning the granular media to the state in which it forms a layer of granular media (14) of the suspended material previously retained in the layer of granular media (14); and - resuming the filtering.

Description

Filtration process for water treatment

[0001] The invention relates to water treatment, and more particularly to the filtration of water to be treated by means of a filter comprising at least one granular media.

[0002] It is known to use one or more filters comprising one or more granular media forming one or more granular layers (or granular bed) in a tank as part of a water treatment process. This may be a wastewater treatment, a treatment intended to make water potable or a seawater desalination process. For example, in the case of wastewater treatment, it is known to carry out tertiary filtration of the wastewater with a granular layer comprising granular activated carbon, i.e. filtration carried out after biological treatment of the wastewater, in order to eliminate suspended matter and micropollutants present in the water. Filtration using granular media may, for example, be implemented in the case of seawater desalination, taking the form of pretreatment of the water before production of fresh water.

[0003] The granular media typically used are sand (more or less fine grain), anthracite, pumice, limestone, gravel or granular activated carbon. Depending on the desired result, for example the nature and size of the suspended matter present in the water to be treated, it is possible to install filters adapted to the latter. Another example is the use of a filter with two granular media (sand / anthracite) for the pretreatment of seawater upstream of the desalination stage.

[0004] Granular media filters will become loaded with particles during their operation. In particular, in the case of a so-called "downflow" granular filter, i.e. in which the current is descending, filtration is mainly done at the surface. Only a small thickness of the granular media will accumulate the matter, which leads to a very localized increase in pressure. This is particularly problematic when the water is heavily loaded with suspended matter or when the filtration speed is high, because this quickly leads to a significant pressure drop. This same phenomenon exists in the lower part of so-called "upflow" granular filters, i.e. with an ascending current.

[0005] These phenomena require cleaning at regular intervals and / or when the filtration capacities of the filters are below a threshold value. The frequency of filter cleaning is more important for wastewater treatment because the latter are more loaded with suspended particles, which leads to the appearance of areas of the filter with a significant pressure drop. This also limits the acceptable concentration of suspended particles in the water to be filtered, which must not be too high in order to allow acceptable operation of the filter in terms of washing frequency while limiting water losses.This can also pose a problem when we know that the wastewater leaving biological treatment (for example treated by an activated sludge process and passing through a clarifier) ​​has a concentration of suspended particles of the order of 5-25 mg / L whereas the operational limit of the filters used in tertiary filtration and comprising granular activated carbon is rather between 5 and 10 mg / L of suspended particles in the filtered water, that is to say downstream of a tertiary filtration stage (granular or on fabric or on membranes).

[0006] Cleaning is achieved by one or more injections of a fluid, typically air and / or water, and rinsing of the granular media, typically with water, before the rinsing water is discharged through a dedicated conduit.

[0007] These cleanings, although necessary, generate certain disadvantages. First of all, cleaning the filters requires them to be stopped. Combined with the filter cleaning time of approximately 25 to 45 minutes, this leads to long shutdowns of the facility. This leads to capping the filtration capacity: either the flow rate of water treated by the filters is high but the cleaning frequency is also high, or the cleaning frequency is reduced by reducing the volumes of water treated over a given period. In addition, each cleaning leads to a loss of water (discharged rinse water), which is initially clear water, and which can eventually be sent to a suitable treatment.However, these cleaning steps of considerable duration are considered necessary because it is commonly accepted that, to have a real effect, cleaning must last a certain time and that it is necessary to evacuate the material accumulated in the filter.

[0008] The invention aims in particular to provide a filtration method using a granular filter as described above making it possible to optimize the operation of the filters while limiting water losses due to cleaning the latter and periods of unavailability.

[0009] For this purpose, the subject of the invention is a method for filtering water to be treated comprising suspended matter using a water filtration device comprising at least one filter comprising at least one granular media forming a submerged granular media layer and placed in a filter reservoir, the filtration method comprising at least two steps of cleaning the filtration device each comprising at least one suspension of a bed of grains forming the granular media by injecting at least one cleaning fluid into the granular media layer, at least one rinsing of the granular media and at least one evacuation of rinsing water comprising suspended matter previously retained in the granular media layer,

[0010] the filtration method comprising at least one step of unclogging the granular media carried out between the two cleaning steps, the unclogging step comprising the following steps: stopping the filtration of the water, at least one suspension of the grains forming the granular media by injecting at least one unclogging fluid into the layer of granular media, stopping the injection of at least one unclogging fluid into the layer of granular media and returning the granular media to the state of a layer of granular media with the suspended matter previously retained in the layer of granular media being fully retained within the tank, and resuming the filtration.

[0011] Thus, between cleaning steps such as described above, one or more filter unclogging steps are carried out, consisting of injecting an unclogging fluid with the filtered impurities remaining in the tank, thus remaining in the granular media, and therefore without evacuating rinsing water, and over very short periods (0.5 to 5 min). Each unclogging operation makes it possible to break one or more zones of the granular media layer in which impurities have concentrated, by applying strong turbulence causing the grains to move and cause friction, such zones forming zones in which the pressure drop is significant.Even if the impurities remain present, they are distributed more uniformly within the granular media layer, which makes it possible to reduce local fouling and therefore local pressure drop in order to regain suitable filtration capacities without having to carry out cleaning, or at least by postponing cleaning to a later date. This makes it possible: to space out the cleaning steps for a volume of water treated per unit of time equivalent to the prior art, and / or to increase the volume of water treated per unit of time and / or to filter water more loaded with suspended particles while maintaining an unchanged cleaning frequency.

[0012] In all cases, at equal concentration of suspended matter, the cleaning frequency per volume of water treated is lower than that of the prior art. The duration of a complete unclogging step being much lower than that of cleaning (approximately 1 to 5 minutes per unclogging compared to 25 to 45 minutes per cleaning), the downtime of the structure, i.e. stopping the production of filtered water, is drastically reduced. In addition, the reduction in the cleaning frequency per volume of water treated, lower than that of the prior art, leads to less use of clear water to be evacuated once soiled, and therefore to be reprocessed.The impurities located on the surface of the granular media before initial cleaning are distributed uniformly over the height of the granular media layer after cleaning, which makes it possible to extend the duration of the effectiveness of the granular media by using the granular media as a whole as a uniform storage space for the impurities. Since washing is less frequent and involves a larger mass of accumulated material, the washing water is more concentrated in material, which facilitates its subsequent reprocessing.

[0013] Finally, regular or even intense mixing of the granular media ensures regular homogenization and therefore homogeneous aging of the latter, in addition to limiting the appearance of preferential filtration paths in the granular media. In addition, regular homogenization of the media promotes aerobic biological activity and prevents the formation of non-aerated zones within the media.

[0014] According to other optional characteristics of the filtration process taken alone or in combination: a height of the granular media layer is between 1 and 4 meters; the filtration of the water is a gravity filtration or a filtration of pressurized water to be treated; the unclogging fluid is injected under the granular media layer and preferably through a perforated support allowing the passage of the unclogging fluid and preventing the passage of the granular media; the injected unclogging fluid is chosen from a gaseous fluid, for example air, water and a mixture of the two; the unclogging fluid is injected for a period between 0.5 and 4 minutes, preferably between 1 and 3 minutes;at least one injection of unclogging fluid corresponds to an injection of air at a speed of between 40 and 60 Nm / h, preferably equal to 55 Nm / h;at least one injection of unclogging fluid corresponds to an injection of water at a speed of between 5 and 40 m / h;the unclogging step is triggered when a parameter representative of a loss of filtration efficiency of the granular media layer reaches a threshold value;a cleaning step is triggered after a predetermined number of unclogging steps or when the duration between two successive unclogging steps is less than a threshold value; andthe water to be treated is chosen from wastewater to be treated in a wastewater treatment plant, water to be made potable in a drinking water plant or seawater to be treated in a desalination plant.

[0015] The invention also relates to a water filtration device comprising: A filter formed from a reservoir comprising at least one granular media forming a layer of granular media immersed and placed in the reservoir, at least one member for supplying water to be treated into the reservoir and at least one member for outlet of treated water from the reservoir, the layer of granular media being arranged between the members for supplying water to be treated and the outlet of treated water, at least one channel for discharging rinsing water from the granular media, at least one member for injecting a fluid for unclogging and / or cleaning the granular media, at least one unclogging control unit configured to trigger a step of unclogging the filter comprising the following steps: stopping the filtration of the water, at least one suspension of the grains forming the granular media by injecting at least one unclogging fluid into the layer of granular media,stopping the injection of at least one unclogging fluid into the granular media layer and returning the granular media to the state of a granular media layer with complete retention within the tank of the suspended matter previously retained in the granular media layer, and resuming filtration.,

[0016] According to other optional characteristics of the filtration device taken alone or in combination: the filtration device comprises at least one unit for measuring at least one parameter representative of a loss of filtration efficiency of the granular media layer, the unit for controlling the triggering of a filter unclogging step being configured to trigger an unclogging step when the parameter representative of a loss of filtration efficiency of the granular media layer reaches a threshold value;the filtration device comprises at least one cleaning control unit configured to trigger a filter cleaning step comprising the following steps: at least one suspension of grains forming the granular media by injecting at least one cleaning fluid into the layer of granular media, at least one rinsing of the granular media and at least one evacuation of a rinsing fluid comprising suspended matter previously retained in the layer of granular media; at least one unclogging fluid injection member is placed so as to inject the unclogging fluid under the layer of granular media; the reservoir comprises a perforated support, the perforations being sized to allow the passage of the unclogging fluid and prevent the passage of grains forming the granular media;the filtration device further comprises a member for injecting new grains of granular media and a member for extracting grains of the granular media present in the tank; andthe filtration device comprises a single granular media forming a single granular layer, or several granular media forming several layers of granular media superimposed in a filtration direction, the granular media being chosen from sand, anthracite, granular activated carbon or any other adsorbent in granular form, pumice stone, limestone or a combination thereof; andthe filtration device forms a device for filtering wastewater to be treated in a wastewater treatment plant, raw water to be treated in a drinking water plant or seawater to be treated in a desalination plant.; Brief description of the figures

[0017] The invention will be better understood on reading the following description, given solely by way of example and with reference to the appended drawings in which:

[0018] is a representation of a filtration device according to a variant of a first embodiment of the invention,

[0019] is a schematic representation of a maintenance method according to the invention,

[0020] is a curve illustrating the evolution of the pressure drop of a granular filter by application of the maintenance method according to the invention and according to a first filtration speed and a first concentration of matter suspended in the water to be treated, and

[0021] is a curve illustrating the evolution of the pressure drop of a granular filter by application of the maintenance method according to the invention and according to a second filtration speed and a second concentration of matter suspended in the water to be treated, and

[0022] is a comparison of a filter according to the prior art with a filter according to the invention. Detailed description

[0023] Reference is now made to the illustration of a water filtration device 22 according to a variant of a first embodiment of the invention. The latter forms in the present example a device for filtration of wastewater to be treated in a wastewater treatment plant. As will be seen subsequently, the invention can also be implemented in the context of the treatment of raw water to be treated in a drinking water plant or seawater to be pretreated in a desalination plant.

[0024] The variant corresponds to filtration from the top to the bottom of the filter in a vertical direction, in other words the filtration is carried out in a descending direction, or "downflow". It can be gravity filtration or pressurized water. The principle of downflow filtration is known to those skilled in the art and will not be the subject of a detailed description in this description.

[0025] The filtration device 22 comprises a filter 10 formed by a reservoir 18 in which is arranged at least one granular media forming a layer of granular media 14, preferably granular activated carbon for the treatment of wastewater (it would also be possible to use sand for example), immersed in the reservoir 18. As we will see later, the filter 10 can comprise a single granular media forming a single granular layer, or several granular media forming several layers of granular media 14 superimposed on each other in a filtration direction.

[0026] The granular media is for example chosen from sand, anthracite, granular activated carbon or any other granular adsorbent, pumice stone, limestone, expanded clay or a combination of these.

[0027] In the example of the, the tank 18 comprises a layer of granular media 14 resting on a layer of gravel. The layer of gravel allows for the retention of finer granular media such as granular activated carbon.

[0028] The filter could include several layers of granular media stacked on top of each other, forming several filtration levels. In such a case, the layer of granular media furthest from a water inlet to be treated is finer than the one closest to a water inlet to be treated, in order to create an additional filtration level.

[0029] The filtration device 22 further comprises a member for supplying water to be treated 24 (for example an injection nozzle, a pressurized pipe or a gravity channel), in the reservoir 18 and at least one treated water outlet member 26, for example a nozzle or an outlet channel, from the reservoir 18, the layer(s) of granular media 14 being arranged between the members for supplying water to be treated 24 and the treated water outlet 26 in the filtration direction. In other words, and on the, the member for supplying water to be treated supplies water to be treated to the reservoir 18 upstream of the layer of granular media 14, that is to say above the layer of granular media 14, and the treated water outlet member allows the water passed through the layer of granular media 14 to leave the filter 10.

[0030] The filtration device 22 further comprises a discharge channel 28 for rinsing water from the granular media. When the granular media is cleaned as explained above, the rinsing water comprising the suspended matter extracted from the granular media must be discharged. It is therefore discharged via the discharge channel 28 placed above the layer of granular media 14 (operation described below).

[0031] The filtration device 22 further comprises at least one member for injecting a fluid 30 (i.e. fluid for unclogging and / or cleaning the granular media), for example an injection nozzle. This member may be, as illustrated in, positioned so as to inject the unclogging fluid below the layer of granular media 14, in order to ensure effective unclogging of the granular media. To do this, it is possible for the reservoir 18 to comprise a perforated support 32, for example a perforated floor, the perforations being sized to allow the passage of the unclogging fluid and treated water and to prevent the passage of grains forming the granular media. Thus, unclogging and cleaning of the granular media are ensured without the granular media risking leaving the reservoir 18 with the treated water during a water filtration step.Alternatively, it is possible to inject a gaseous fluid, for example air, into the thickness of the granular media layer 14.

[0032] Generally speaking, unclogging corresponds to an injection of fluid with sufficient force to suspend (or otherwise expand) the granular media and this in such a way as to break up agglomerates of filtered impurities, in particular present in an upper part of the layer of granular media 14.

[0033] As an alternative to using a perforated support, it is possible to use a set of perforated pipes for injecting a cleaning fluid.

[0034] The filtration device 22 further comprises at least one unclogging control unit configured to trigger a step of unclogging the filter 10 as described above. Advantageously, the filtration device 22 may comprise at least one unit for measuring at least one parameter representative of a loss of filtration efficiency of the granular media layer 14, the unit for controlling the triggering of a step of unclogging the filter 10 being configured to trigger an unclogging step when the parameter representative of a loss of filtration efficiency of the granular media layer reaches a threshold value.For example, the unit for measuring at least one parameter representative of a loss of filtration efficiency of the granular media layer 14 may comprise: means for determining a change in the pressure drop of the water passing through the granular media layer 14, such as for example the increase in the water level in the reservoir 18 or the measurement of a pressure difference between the top and the bottom of the filter 10, the change of which indicates a loss of the capacity of the granular media layer 14 to filter the water to be treated, and / or means for measuring a concentration of suspended impurities in the treated water and the flow rate applied to the structure (flow of suspended matter) the increase of which illustrates the loss of capacity of the filter to retain the impurities, and / or means for measuring a time interval since the last unclogging step, and / or means for measuring a flow rate of the filter when the filtration is a pressurized water filtration.

[0035] The filtration device 22 further comprises at least one cleaning control unit configured to trigger a filter cleaning step according to the invention and described above.

[0036] Advantageously, particularly in the case of using granular activated carbon, it is possible to provide the filtration device 22 with a member 34 for injecting new grains of granular media and a member 36 for extracting grains from the granular media present in the reservoir. Thus, it is possible to renew the granular media, but also to compensate for a loss of grains due to cleaning the filter 10, or even to extract a portion of a less effective granular media (for example, grains of activated carbon saturated with adsorbed impurities).

[0037] Reference is now made to the diagram illustrating schematically a filtration method of the filtration device 22 of the first variant described above.

[0038] We will first describe the conventional operation of a filtration step 1 using a filtration device 22 as described above.

[0039] During this filtration step 1, water to be treated, here waste water, is introduced into the tank 18 via the water to be treated supply member 24. This water to be treated is then located upstream of the layer(s) of granular media 14, here granular activated carbon, in a direction of filtration of the water to be treated, here from top to bottom.

[0040] The water to be treated will pass through the granular media 14 which will retain impurities present in the water to be treated in the form of suspended matter. These may be impurities blocked between the grains of the granular media 14, impurities adsorbed by a granular media having the capacity (for example activated carbon), etc.

[0041] Treated water leaves the granular media layer 14, below the latter in the downflow filtration example, to be collected outside the filter 10 by the treated water outlet member 26. In the example of 1a, the treated water passes through the perforated support 32. As explained above, the latter allows the passage of the treated water and prevents the passage of the granular media.

[0042] Water to be treated continuously arrives from the water to be treated supply member 24, and treated water continuously leaves the treated water outlet member 26.

[0043] As explained above, and in the case of wastewater treatment, the filter 10 may comprise at least one layer of adsorbent, for example granular activated carbon, capable of blocking certain types of impurities and adsorbing other types of impurities.

[0044] When cleaning is necessary, filtration step 1 is stopped to make way for a cleaning step 2. As explained above, the filtration process comprises at least two cleaning steps 2 of the filter 10.

[0045] A cleaning step 2 comprises: At least one suspension of grains forming the granular media by injecting at least one cleaning fluid into the granular media layer 14. In the example of 1a, a cleaning fluid is injected by the fluid injection member 30 below the granular media layer 14 and through the perforated support 32 allowing the passage of the cleaning fluid and preventing the passage of the granular media. The injected cleaning fluid may be a gaseous fluid, for example air, or water, or even a mixture of the two. The injected fluid may for example be air, for example injected at a speed of between 40 and 60 Nm / h for a period of between 0.5 and 4 minutes.Following the suspension of grains forming the granular media 14 by injecting at least one cleaning fluid into the layer of granular media 14, at least one rinsing of the granular media 14 is carried out, conventionally with clean water (typically treated water filtered and stored in a treated water tank in the case of wastewater treatment), in order to separate the impurities from the grains of the granular media. The rinsing with clean water can be carried out for a single period of between 10 and 35 minutes or preferably for two phases of between 1 and 5 minutes and then between 10 and 30 minutes. In this second case, a second phase of suspending the media is carried out between the two rinsing phases, in order to improve the quality of the washing. More generally, it can be said that the total duration of the rinsing is between 10 and 35 minutes. This leads to an increase in the water level in the tank 18.The rinsing water loaded with suspended matter is discharged through the discharge channel 28. The latter may be reprocessed subsequently. Finally, each cleaning step may include a rest step, for example between 30 and 60 seconds, in order to allow the granular media to resume its shape as a layer of granular media 14. This is particularly true for granular activated carbon and more generally for light granular media, the suspension of which corresponds to a significant expansion of the granular media.

[0046] Preferably, the following sequence can be carried out for an optimal cleaning step (after stopping filtration): a first injection of air at a speed of between 40 and 60 Nm / h for a period of between 0.5 and 4 minutes, a first rinse with clean water for 1 to 5 minutes, a second injection of air at a speed of between 40 and 60 Nm / h for a period of between 0.5 and 4 minutes, a second rinse with clean water for 10 to 30 minutes.

[0047] The steps described above may be preceded by a step of lowering the water level, for example 20 centimeters below the water extraction point to anticipate the rise in the water level and the high turbulence created by the injection of air, in order to avoid the loss of media in the dirty water recovery member. Here again, this is particularly relevant for light granular media such as granular activated carbon, because a rise in the water level without prior lowering increases the risk of loss of granular media in this case by bringing the granular media to the level of the discharge pipe 28. They are followed by a rest step of 30 to 60 seconds in order to allow the granular media(s), particularly light granular media as explained above, to return to their initial position (i.e. to reform one or more layers of granular media 14) before restarting a filtration step 1.

[0048] Following a cleaning step 2, water filtration can start again, as seen in the. A water supply control valve is opened gradually, and can be opened to 100% during water filtration (step 3 of the).

[0049] According to the invention, the filtration method comprises at least one step 4 of unclogging the granular media carried out between the two cleaning steps 2. In the example of 1a, two steps of unclogging 4 are carried out between two cleaning steps 2.

[0050] Each unclogging step 4 begins with a stoppage of a filtration step 1. As explained above, at least one unit for measuring at least one parameter representative of a loss of filtration efficiency of the granular media layer 14 can make it possible to determine the triggering of an unclogging step 4 (step 5 for detecting a need to unclog the filter on the). The various parameters that can be measured can be those described previously. For example, pressure sensors can be used to calculate the pressure drop of the filter, sensors for the water level in the tank or sensors for determining the level of suspended particles (for example, water turbidity sensors) in order to determine whether an unclogging step 4 must be carried out.

[0051] Each unclogging step comprises at least one suspension of the grains forming the layer(s) of granular media 14 by injecting at least one unclogging fluid into the layer of granular media 14. This injection, leading to an expansion of the layer(s) of granular media 14 to a greater or lesser extent depending on the nature of the latter, may be similar to that(s) carried out during a cleaning step 2, both in terms of the nature of the fluid injected (water, air, air / water mixture, etc.) and in terms of the injection method (for example below the layer of granular media 14 and through a perforated support 32).

[0052] The injection of the unclogging fluid is an upflow injection, regardless of the direction of filtration of the water to be treated.

[0053] The fluid injected during the unclogging step 4 can therefore be water and / or air. In the case of wastewater treatment, an injection of air alone is preferable. Indeed, the accumulated matter is relatively sticky, mainly biological residues and exopolymers, and the injection of air makes it possible to effectively break the clogging. The advantage of using air is, in addition to effective unclogging, to regularly supply the granular media with air, which makes it possible to promote aerobic biological activity in the filter 10 and to avoid maintaining certain areas in anoxic or anaerobic conditions for too long (presence of a large quantity of filtered matter (by limiting the cleaning steps 2) and regular supply of oxygen).

[0054] At least one injection of fluid may correspond to an injection of air at a speed of between 40 and 60 Nm / h, preferably equal to 55 Nm / h. This is an injection speed sufficient to obtain unclogging of the zones of the filter 10 having a significant concentration of impurities, and corresponding to the zones of the filter 10 where the pressure drop is the greatest.

[0055] At least one fluid injection can correspond to an air injection for a period of between 0.5 and 4 minutes. Here again, this is a sufficient injection time to obtain unclogging of the filter areas with a high concentration of impurities, and corresponding to the filter areas where the pressure drop is greatest.

[0056] In the case of water injection, at least one fluid injection corresponds to a water injection at a speed of between 5 and 40 m / h. Again, this is a sufficient injection time to obtain unclogging of the filter areas with a high concentration of impurities, and corresponding to the filter areas where the pressure drop is greatest.

[0057] Finally, at least one fluid injection may correspond to a water injection for a period of between 0.5 and 4 minutes, preferably between 1 and 3 minutes, which may correspond to an expansion of the granular media 14 of 5 to 30%, for an injection speed of between 5 and 40 m / h. Here too, this is a sufficient injection time to obtain unclogging of the areas of the filter having a significant concentration of particles, and corresponding to the areas of the filter where the pressure drop is the greatest.

[0058] It is possible to provide that at least one fluid injection comprises at least one injection of a mixture of a gaseous fluid and a liquid fluid, for example a mixture of water and air.

[0059] The choice of an injection of water and / or air (or another fluid) as well as the injection sequence(s) or even the injection parameters (injection time and injection speed) can be chosen according to the nature of the granular media (for example the size of the grains), the type of clogging material which itself depends on the application (untreated wastewater, biologically treated wastewater, surface water, seawater, etc.), the level of clogging of the filter 10, etc. In particular, and in the case of wastewater treatment, the unclogging fluid is preferably water or a mixture of water and air to generate sufficient turbulence and friction between the grains of granular media and carry out effective unclogging.

[0060] The injection of at least one fluid into the granular media may be preceded by a lowering of a water level in the reservoir 18 relative to a water level at the time of stopping the filtration step 1, for the reasons mentioned above.

[0061] Each unclogging step 4 comprises a step of stopping the injection of at least one unclogging fluid into the granular media layer 14 and returning the granular media to the state of granular media layer 14 with complete retention within the tank of the suspended matter previously retained in the granular media layer 14. This return step may therefore correspond to a rest of 30 to 60 seconds (especially for granular activated carbon) in order to allow the granular media to return to its initial position (i.e. layer(s) of granular media 14) before restarting filtration.

[0062] During this return to the initial position, the suspended matter already filtered is distributed in a substantially homogeneous manner in the granular media, more particularly in the thickness of the granular media. In other words, the impurities which formed blocks hindering filtration in the upper part of the granular media layer 14 are found distributed in a more diffuse and homogeneous manner throughout the thickness of the granular media layer 14, which makes it possible to improve the filtration capacities of the latter without having to carry out a complete cleaning. As explained above, this makes it possible: to extend the filtration time between two washing steps 2, and / or to treat water more loaded with suspended matter, and / or to treat water having an unchanged concentration of suspended matter but where coagulation is accepted to trap more soluble matter in addition to the suspended matter, and / or to increase the filtration speed.

[0063] Following one or more unclogging steps 4, a filtration step can be started again as described above. In the example of the two unclogging steps 4 are implemented between two cleaning steps 2, a filtration step 1 being repeated after each unclogging step 4.

[0064] A cleaning step 2 can be triggered after a determined number of unclogging steps 4 (it is possible for certain applications to determine the maximum number of unclogging steps 4 before imposing a cleaning step 2) or when the duration between successive unclogging steps 4 is less than a threshold value (in other words when the unclogging steps 4, when triggered by measuring a parameter representative of the capacity of the filter 10 to treat the water, are too close to each other), or even after a time limit since the last washing step to impose a minimum washing frequency (for example to limit the residence time of the impurities in the filtration device 22) (step 7 of detecting a need to clean the filter on the).

[0065] As explained above, the filter may at least partially form a primary or tertiary filtration device of a wastewater treatment assembly. According to a first example of filtration being a tertiary filtration of wastewater via the granular media layer, here a granular activated carbon filter, each unclogging step 4 may comprise, by way of example only, the following steps: Detection of a need for unclogging by at least one of the detection possibilities described above. Stopping the filtration. Lowering the water level inside the tank 18 as described above. Injecting air for 0.5 to 4 minutes at a speed of 55 Nm / h. Resting the filter 10 in order to let the granular media 14 return to its initial position as a granular media layer 14 before restarting a filtration.

[0066] Steps 3 to 5 can be repeated several times before resuming filtration.

[0067] Figures 3 and 4 represent curves illustrating the evolution of the pressure drop of a granular filter of the type of that of the invention, comprising a layer of granular activated carbon, by application of the filtration method according to the example above. Illustrates a filtration at a speed of 10 m / h of water whose total concentration of suspended matter is between 7 and 10 mg / L. Illustrates a filtration at a speed of 15 m / h of water whose total concentration of suspended matter is between 6.6 and 10.2 mg / L.

[0068] A cleaning step 2 is carried out when the pressure drop reaches a predetermined threshold value representing the pressure drop value from which cleaning of the filter 10 is necessary. It can be seen that other cleaning steps 2 are carried out approximately every 36 hours for the same reasons.

[0069] Several unclogging steps 4 are carried out between the two cleaning steps 2. A unclogging 4 is carried out approximately every 2 hours and 30 minutes in the examples illustrated. It is observed that each unclogging 4 makes it possible, even with the suspended matter filtered by the layer or layers of granular media remaining completely within the tank, to reduce the overall pressure drop of the filter.

[0070] Lines 8 represent a projection of the evolution of the overall pressure drop of the filter if no unclogging step 4 was carried out. It makes it possible to visualize that, without unclogging, cleaning would have been necessary approximately 10-12 hours after the first cleaning step 2, compared to approximately 36 hours when implementing the filtration method according to the invention. It is even possible to advance that, with equivalent and constant operation, cleaning would be necessary every 10-12 hours. By combining this measurement with the downtime of the filtration device 22 for cleaning as well as the quantities of water used for each cleaning, the interest of the method according to the invention is easily understood.As explained previously, the extension of the acceptable operating range of the filter 10 makes it possible, alternatively or in addition to a spacing of the cleaning steps 2, to treat water at a higher speed and / or having a higher level of suspended matter than according to the prior art.

[0071] Illustrates on the left a 10' filter according to the prior art and on the right a 10'' filter maintained by implementing a method according to the invention. On the left, two layers of granular media 14 are present (a finer layer in the upper part and a coarser layer in the lower part) and have retained a layer of impurities which are concentrated on the upper part 16 of the upper layer of granular media 14. The filter 10' sees its water filtration capacity reduced (arrow 15 illustrating the loss of filtration capacity of the filter 10'), which leads to an increase in the pressure drop which can lead to a rise in the water level in the reservoir 18 (arrows 17 showing the rise in the water level).In the figure on the right, and after one or more unclogging steps 4 according to the invention, the filtered impurities are distributed uniformly in at least one layer of granular media 14, here the upper layer, which allows the filter 10'' to maintain an acceptable filtration capacity (arrow 20 for water filtration) allowing it to continue to operate without the need for complete cleaning of the layer(s) of granular media 14.

[0072] According to a second variant of the first embodiment of the invention also relating to the treatment of wastewater, it is possible to carry out filtration in an upward direction, that is to say from the bottom to the top in a substantially vertical direction, or "upflow". Only the differences with the first variant will be described for this second variant.

[0073] In this variant, the injection of unclogging fluid, carried out in an upward direction, is therefore carried out in the same direction as the filtration direction. This filtration is preferably carried out in a limited speed range, typically less than or equal to 10 m / h, mainly to avoid the risk of lifting the granular media layer and to maintain effective filtration. It is less sensitive to clogging than downflow filtration because the suspended matter penetrates more deeply into the granular media. Alternatively, the filtration is carried out at a speed greater than 10 m / h. In this case, the granular media expands and the filtration is less effective. Clogging is therefore less significant.

[0074] Upward filtration can eliminate the need for a perforated media, although one can be used. For example, the water to be treated can be conveyed through one or more perforated pipes.

[0075] Upflow filtration limits the water height above the granular media (because the water arrives below the latter and the water above is filtered water and evacuated from the filtration device as filtration progresses). The size of the filtration device can therefore be reduced. Indeed, the water height above the granular bed in a "downflow" type filter allows for the generation of a thrust pressure in the water column. This limitation does not exist in "upflow" operation. The water height at the surface of the granular bed is chosen to avoid loss of the granular media in the dirty wash water.

[0076] The presence of a coarse gravel-type media can be interesting, in addition to its role as a support for a granular media located above it, to distribute the water to be treated arriving below it in the granular media. Alternatively to the presence of gravel fulfilling this role, water injection nozzles to be treated can be used to inject the water to be treated into the granular media layer in a homogeneous manner.

[0077] Upflow filtration can be achieved with multiple layers of granular media, for example with two different layers of granular activated carbon.

[0078] According to a second embodiment of the invention, the filtration device and the filtration method are implemented for the desalination of seawater, in particular for pretreatment of seawater before desalination, for example by reverse osmosis. This embodiment includes all the elements described above, only the differences are described below.

[0079] According to this embodiment, the choice of the granular media(s) present in the tank can be adapted to the filtration of seawater. For example, it is possible to put in place two layers of granular media superimposed in a filtration direction, one layer being formed of anthracite and the other layer being formed of sand.

[0080] Downflow filtration is preferred for seawater desalination.

[0081] Water is the most suitable unclogging fluid for carrying out unclogging in this embodiment, air can be considered (even if oxygenation of the water is not sought in this application).

[0082] The presence of granular media grain injection and granular media grain extraction members is not required in this embodiment.

[0083] Finally, in the case of desalination, it is possible to use brines (treated water from a desalination process and concentrated in salt) as a cleaning fluid. Seawater is used as rinsing water.

[0084] According to an example of a seawater filtration process given solely as an example, a pretreatment of seawater is carried out via an anthracite / sand filter as described above. In order to unclog the filter, the unclogging step 4 is as follows: Detection of a need for unclogging by at least one of the detection possibilities described above. Stopping filtration. Lowering the water level inside the tank. Injection of water countercurrent to one filtration direction for 30 seconds to 2 minutes at a speed substantially equal to 35 m / h. Possible rest of the filter in order to allow the granular media to return to its initial position before restarting filtration. However, a quasi-instantaneous restart of filtration is possible, the return of the granular media to the state of a granular media layer following an injection of unclogging fluid being quasi-instantaneous.

[0085] According to a third embodiment of the invention, the filtration device and the filtration method are implemented for the treatment of drinking water. This embodiment includes all the elements described for the first embodiment of the invention, only the differences are described below.

[0086] In the context of drinking water treatment, the presence of granular media grain injection and granular media grain extraction devices is not required.

[0087] Downflow filtration is preferred for drinking water treatment, although upflow filtration is possible. List of references

[0088] 1: filtration step

[0089] 2: cleaning step

[0090] 3: complete opening of a regulating valve

[0091] 4: unclogging step

[0092] 5: detection of a need to unclog the filter

[0093] 7: detection of a need to clean the filter

[0094] 8: sealing projection

[0095] 10, 10', 10'': filters

[0096] 14: granular media layer(s)

[0097] 15: loss of filtration capacity

[0098] 16: upper part of the granular media

[0099] 17: water level rise

[0100] 18: tank

[0101] 20: water filtration

[0102] 22: filtration device

[0103] 24: water supply organ to be treated

[0104] 26: treated water outlet organ

[0105] 28: drainage channel

[0106] 30: fluid injection organ

[0107] 32: perforated support

[0108] 34: granular media grain injection organ

[0109] 36: granular media grain extraction organ

Claims

A method of filtering water to be treated comprising suspended matter using a water filtration device (22) comprising at least one filter (10, 10'') comprising at least one granular media forming a submerged granular media layer (14) and placed in a reservoir (18) of the filter (10, 10''), the filtration method comprising:at least two cleaning steps (2) of the filtration device (22) each comprising at least one suspension of the grains forming the granular media by injecting at least one cleaning fluid into the granular media layer (14), at least one rinsing of the granular media and at least one evacuation of rinsing water comprising suspended matter previously retained in the granular media layer (14),the filtration method being characterized in that it comprises at least one unclogging step (4) of the granular media carried out between the two cleaning steps (2),the unclogging step comprising the following steps: stopping the filtration of the water, at least one suspension of the grains forming the granular media by injecting at least one unclogging fluid into the layer of granular media (14), stopping the injection of at least one unclogging fluid into the layer of granular media and returning the granular media (14) to the state of a layer of granular media (14) with the suspended matter previously retained in the layer of granular media (14) being completely retained within the tank, and resuming the filtration., A filtration method according to claim 1, wherein a height of the granular media layer (14) is between 1 and 4 meters. Filtration method according to any one of the preceding claims, in which the unclogging fluid is injected under the layer of granular media (14) and preferably through a perforated support (32) allowing the passage of the unclogging fluid and preventing the passage of the granular media. Filtration method according to any one of the preceding claims, in which the injected unclogging fluid is chosen from a gaseous fluid, for example air, water and a mixture of the two. Filtration method according to claim 4, in which the unclogging fluid is injected for a period of between 0.5 and 4 minutes, preferably between 1 and 3 minutes. Filtration method according to any one of claims 4 or 5, in which at least one injection of unclogging fluid corresponds to an injection of air at a speed of between 40 and 60 Nm / h, preferably equal to 55 Nm / h. Filtration method according to any one of claims 4 or 5, in which at least one injection of unclogging fluid corresponds to an injection of water at a speed of between 5 and 40 m / h. Filtration method according to any one of the preceding claims, in which the unclogging step (4) is triggered when a parameter representative of a loss of filtration efficiency of the granular media layer reaches a threshold value. Filtration method according to any one of the preceding claims, in which a cleaning step (2) is triggered after a predetermined number of unclogging steps (4) or when the duration between two successive unclogging steps (4) is less than a threshold value. Filtration method according to any one of the preceding claims, in which the water to be treated is chosen from wastewater to be treated in a wastewater treatment plant, water to be made potable in a drinking water plant or seawater to be treated in a desalination plant. A water filtration device (22) comprising: A filter (10, 10'') formed of a reservoir comprising at least one granular media (14) forming a layer of granular media (14) immersed and placed in the reservoir (18), at least one member for supplying water to be treated (24) into the reservoir (18) and at least one member for outlet of treated water (26) from the reservoir (18), the layer of granular media (14) being arranged between the members for supplying water to be treated (24) and the outlet of treated water (26), at least one discharge channel (28) for rinsing water from the granular media, at least one member for injecting (30) a fluid for unclogging and / or cleaning the granular media, at least one unclogging control unit configured to trigger an unclogging step (4) of the filter comprising the following steps: stopping water filtration,at least one suspension of the grains forming the granular media by injection of at least one unclogging fluid into the layer of granular media (14), stopping the injection of at least one unclogging fluid into the layer of granular media and returning the granular media to the state of layer of granular media (14) with complete maintenance within the reservoir of suspended matter previously retained in the layer of granular media (14), and resuming filtration., Filtration device according to claim 11, comprising at least one unit for measuring at least one parameter representative of a loss of filtration efficiency of the granular media layer (14), the unit for controlling the triggering of an unclogging step (4) of the filter (10, 10'') being configured to trigger an unclogging step (4) when the parameter representative of a loss of filtration efficiency of the granular media layer (14) reaches a threshold value. Filtration device according to any one of claims 11 or 12, comprising at least one cleaning control unit configured to trigger a cleaning step (2) of the filter (10, 10'') comprising the following steps: at least one suspension of grains forming the granular media by injection of at least one cleaning fluid into the layer of granular media (14), at least one rinsing of the granular media and at least one evacuation of a rinsing fluid comprising suspended matter previously retained in the layer of granular media (14). Filtration device according to any one of claims 11 to 13, in which at least one unclogging fluid injection member (30) is placed so as to inject the unclogging fluid under the layer of granular media (14). Filtration device according to any one of claims 11 to 14, comprising a single granular media (14) forming a single granular layer, or several granular media (14) forming several layers of granular media (14) superimposed in a filtration direction, the granular media being chosen from sand, anthracite, granular activated carbon or any other adsorbent in granular form, pumice stone, limestone or a combination thereof.

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